Computer systems comprising one or more processors in communication with primary storage often called “main memory” or “main store” execute programs and manipulate data residing in main store. Computer systems compensate for the small amount off main store by providing virtual store wherein data containers, called “pages” of the virtual store are backed in a “page file” on a hard disk drive. In order to make the virtual store available to the processor, current pages of virtual store are backed by pages of main store accessible to the processor. As main memory fills up, pages are pushed out of main memory and, if modified, saved in the secondary storage system for later retrieval if necessary. If the page has not been modified only the reference to the page is updated to reflect that it is located on the secondary store. This process is called “page swapping” and if it is frequent, a serious degradation of performance ensues. A “page fault” occurs when a page is requested by the operating system or application that is not resident in the processor's main memory (main store). Typical solutions to this problem include better, more advanced algorithms for memory management and more simply increasing the amount of main memory available to the processor.
Serving pages out of the hard disk page file degrades system performance, especially with mobile devices where power conservation is important. Current mobile devices with hard disk drives suffer the most from the traditional virtual memory system. Disk drives in the present specification will be used to refer to a variety of permanent storage devices including but not limited to mechanically operated magnetic storage devices (disks, tapes), CD's, DVD's and the like. Mobile devices conserve power by shutting off unneeded components of the system. During long periods of disk inactivity, the hard disk drive may be powered off to prolong disconnected operation and conserver battery power. If a page fault occurs when the drive is powered off a noticeable delay ensues while the old page is “paged out” to the drive or while the new page is loaded from the drive in addition to the increased power consumption. Mechanical permanent store is slow even when powered on in that there is considerable mechanical delay in seeking the page to be loaded. If a page is frequently paged, the performance effect of waiting for a mechanical disk to power up or just to seek the page is more keenly felt.
U.S. Pat. No. 6,438,668: “Method and apparatus for reducing power consumption in a digital processing system” filed Sep. 30, 2002 incorporated herein by reference, discusses methods and apparatuses for controlling power consumption in a digital processing system. In one aspect of the invention, an exemplary method includes using a non-volatile memory of the digital processing system (DPS) as a virtual memory of a volatile random access memory (RAM) of the DPS, determining a selection of a reduced power consumption state, storing, in response to the selection and through a virtual memory process, data from the volatile RAM to the non-volatile memory, and reducing power to at least one element of the data processing system after the storing, wherein the storing includes determining whether to store the data to the non-volatile memory by determining if first data previously stored as virtual memory in the non-volatile memory is valid (not dirty) after the selection.
U.S. Pat. No. 5,778,443: “Method and apparatus for conserving power and system resources in a computer system employing a virtual memory” filed Dec. 14, 1994 incorporated herein by reference discloses a computer system that has volatile random access memory (“RAM”) and nonvolatile auxiliary storage, a virtual memory operating system with some pages of virtual memory resident in RAM and other pages resident in a paging space in auxiliary storage. A time varying operating state for the computer is definable by reference to contents of the memory. A space in auxiliary storage (a “hibernation space”) is allocated for storing a portion of RAM as a hibernation image. A first group of the RAM-resident virtual memory pages is stored in the paging space. A second group of the RAM-resident virtual memory pages is stored in the hibernation space. A hibernation state is entered where the computer system is powered off with the system at a certain operating state. The computer system is returned to operation at the certain operating state, which includes powering on the computer system and reading the second group of pages into the RAM.
U.S. Pat. No. 5,732,238: “Non-volatile cache for providing data integrity in operation with a volatile demand paging cache in a data storage system” filed Jun. 12, 1996 incorporated herein by reference discloses a non-volatile cache mechanism connected to a bus connected for conducting write addresses and data from a host computer to mass storage devices and to a volatile cache wherein each write operation includes a write address and at least one data word. The non-volatile cache mechanism includes a non-volatile memory constructed of a plurality of sub-memories having overlapping read/write cycles for storing the data words, a cache control responsive to the write operations for writing the data words into the non-volatile memory in parallel with receipt of the data words into the volatile cache, and a cache index for storing index entries relating write addresses of write operations on the bus with corresponding storage addresses of the data words in the non-volatile memory. The cache control is responsive to a write operation for reading the index entries to identify and select at least one available storage address in the non-volatile memory, generating at least one index entry relating the write address of the current write operation and the selected storage addresses in the non-volatile memory, and writing the data words into the non-volatile memory. The cache control is responsive to flush addresses to the volatile cache for indexing the cache index to identify cache entries corresponding to the flush addresses and invalidating the corresponding cache entries.
U.S. Pat. No. 5,390,334: “Workstation power management by page placement control” filed Nov. 9, 1992 incorporated herein by reference discloses a power conserving method and apparatus for managing a computer memory. A first memory bank comprises normal RAM. A second memory bank comprises low-power RAM. More frequently used virtual pages are mapped onto the first memory bank while less frequently used virtual pages are mapped onto the second memory bank. Further power savings are achieved by removing power from the driver circuits of the memory controller for the second memory bank when the second memory bank is not being referenced.
Performance degradation due to traditional paging of data between permanent store and system main memory particularly in mobile devices adversely effects the usefulness of the computer system. A method is needed to improve paging performance.